This can be how you look at the numbers. You can look at the numbers like this:
1, 2, 3, 4, 5, | 6, 7, 8, 9, 10
where the half-way mark is on the right of five.
Or you can look at it like this:
0, 1, 2, 3, 4, | 5, 6, 7, 8, 9
where the halfway mark is to the left of the 5 if you move the 0 in the ones place to the other side, which is why 5 will round up.
If you want to have your mind blown (you will likely cover this in the future depending on how far you take your math lessons), the more advanced math will have you round 5 up or down depending on the next place over if it's odd or even.
35 will round up to 40 because 40 is even and 30 is odd.
25 will round down to 20 because 30 is odd and 20 is even.
(It's a division issue--evens are much easier to divide by than odd numbers.)